1. Field of the Invention
This invention relates to screened inductance sensors. The invention relates in particular to printed circuit windings for screened inductance sensors including, but not limited to, level gauge sensors having sealing barriers, and to methods for the production of such windings, as well as to screened inductance transducer geometries in which there is a sealing barrier between the windings and the screen(s), especially screened inductance sensors for liquid level measurement.
2. Description of the Prior Art
A screened inductance sensor is a position measurement device in which inductive coupling between two, normally stationary, coils is controlled by the position of a passive conductive screen.
Position and speed measurement systems based on the screened inductance principle have been described in previous patent applications. In particular, position and speed sensors form the subject of co-pending U.S. patent application Ser. No. 789,147, now allowed. According to this application, there is provided a sensor comprising a drive or exciting winding for establishing a forward or drive field, at least one secondary or sense winding in which a voltage may be induced in the presence of said forward or drive field, and at least one conductive screen within which eddy currents are generated in the presence of said forward or drive field to establish a counter-field opposing said forward or drive field, said at least one conductive screen and said at least one secondary or sense winding being displaceable relative to one another within said forward or drive field so that said at least one secondary or sense winding may be shaded by said at least one conductive screen to a varying extent to thereby vary the voltage induced in said at least one secondary or sense winding, the relative displacement of said at least one conductive screen and said at least one secondary or sense winding being limited to substantially a single direction of displacement, said drive or exciting winding being configured so that in the absence of said at least one conductive screen said forward or drive field is substantially even over said at least one secondary or sense winding in said single direction of relative displacement of said at least one conductive screen and said at least one secondary or sense winding, said at least one secondary or sense winding being a two-terminal winding, and said at least one conductive screen having a surface region which is substantially parallel to a notional surface defined by said at least one secondary or sense winding.
Said surface region of said at least one conductive screen may be substantially planar and said at least one secondary or sense winding may be a substantially flat winding defining a plane substantially parallel to said surface region. Said at least one screen and said at least one secondary or sense winding may then suitably be relatively linearly displaceable, or alternatively, they may be relatively rotationally displaceable.
Said surface region of said at least one conductive screen may be a sector of a cylindrical surface and said at least one secondary or sense winding may be disposed about the periphery of a notional cylinder substantially concentric with said cylindrical sector surface region, said at least one conductive screen and said at least one secondary or sense winding then being relatively displaceable in a circumferential direction about the substantially common axis of said cylindrical surface and said notional cylinder.
In an advantageous construction of sensor according to said co-pending Application there is a further secondary or sense winding, the progressive shading of which during said relative displacement of said at least one conductive screen and said at least one secondary or sense winding proceeds in a manner substantially identical with the progressive shading of said at least one secondary or sense winding during said relative displacement, said further secondary or sense winding being disposed relative to said at least one secondary or sense winding so that the progressive shading of said further secondary or sense winding during said relative displacement is spaced apart from that of said at least one secondary or sense winding, the voltage variations in each said secondary or sense winding during said progressive shading preferably following a cyclic pattern and said further secondary or sense winding being disposed relative to said at least one secondary or sense winding so that said voltage variations are in phase quadrature.
In a favoured winding arrangement, said at least one secondary or sense winding has first and second coils, said coils being connected so that in the presence of said forward or drive field the voltage induced in the second coil in the absence of said at least one conductive screen is substantially equal in magnitude but opposite in polarity to the voltage induced in the first coil, said at least one secondary or sense winding preferably having a plurality of coils extending in said single direction of relative displacement and each successive coil in said single direction of relative displacement being wound in the opposite electrical sense to each adjacent coil.
In an especially favoured arrangement, said at least one secondary or sense winding has at least one coil having a pitch or extent in said single direction of relative displacement and said at least one conductive screen has a pitch or extent in said single direction of relative displacement which is substantially equal to the pitch or extent of said at least one coil, said at least one conductive screen having a dimension in a direction at right angles to said single direction of relative displacement which is substantially constant over said pitch or extent of said at least one conductive screen and said at least one secondary or sense winding having a dimension in a direction at right angles to said single direction of relative displacement which is substantially constant over said pitch or extent of said at least one coil, said screen dimension in said direction at right angles to said single direction of relative displacement being optionally substantially equal to the maximum dimension of said at least one secondary or sense winding in said direction at right angles to said direction of relative displacement.
Said at least one secondary or sense winding may have at least one coil having a pitch or extent in said single direction of relative displacement and said at least one conductive screen may have a pitch or extent in said single direction of relative displacement which is substantially different from the pitch or extent of said at least one coil, said at least one secondary or sense winding having a dimension in a direction at right angles to said single direction of relative displacement which changes progressively over said pitch or extent of said at least one coil.
Finally, there is also provided according to said co-pending Application, a sensing system comprising a sensor having a drive or exciting winding for establishing a forward or drive field, at least one secondary or sense winding in which a voltage may be induced in the presence of said forward or drive field, and at least one conductive screen within which eddy currents are generated in the presence of said forward or drive field to establish a counter-field opposing said forward or drive field, said at least one conductive screen and said at least one secondary or sense winding being displaceable relative to one another within said forward or drive field so that said at least one secondary or sense winding may be shaded by said at least one conductive screen to a varying extent to thereby vary the voltage induced in said at least one secondary or sense winding, said relative displacement of said at least one conductive screen and said at least one secondary or sense winding being limited to substantially a single direction of displacement, said drive or exciting winding being configured so that in the absence of said at least one conductive screen said forward or drive field is substantially even over said at least one secondary or sense winding in said single direction of relative displacement of said at least one conductive screen and said at least one secondary or sense winding, said at least one secondary or sense winding being a two-terminal winding, and said at least one conductive screen having a surface region which is substantially parallel to a notional surface defined by said at least one secondary or sense winding, means for applying an alternating voltage to said drive or exciting winding, and means for processing the output signal from said at least one secondary or sense winding to provide a signal indicative of the relative disposition of said at least one conductive screen and said at least one secondary or sense winding.
Said co-pending application also discloses a sensor comprising a substantially solenoidal drive or exciting winding for establishing a forward or drive field, at least one secondary or sense winding in which a voltage may be induced in the presence of said forward or drive field, and at least one conductive screen within which eddy currents are generated in the presence of said forward or drive field to establish a counter-field opposing said forward field, said solenoidal drive or exciting winding and said at least one secondary or sense winding being substantially coaxial, said at least one conductive screen and said at least one secondary or sense winding being displaceable relative to one another within said forward or drive field so that said at least one secondary or sense winding may be shaded by said at least one conductive screen to a varying extent to thereby vary the voltage induced in said at least one secondary or sense winding, said at least one conductive screen having an axis of symmetry and a surface region which extends substantially circumferentially with respect to said axis of symmetry, said at least one secondary or sense winding being disposed about the periphery of a notional cylinder which is substantially coaxial with said axis of symmetry of said at least one conductive screen, and the relative displacement of said at least one conductive screen and said at least one secondary or sense winding being in the direction of said axis of symmetry of said at least one conductive screen and said axis of said notional cylinder which is substantially coaxial therewith.
Said at least one secondary or sense winding may have first and second coils, said coils being connected so that in the presence of said forward or drive field the voltage induced in the second coil in the absence of said at least one conductive screen is substantially equal in magnitude but opposite in polarity to the voltage induced in the first coil.
Said surface region of said at least one conductive screen may be substantially circular cylindrical and said at least one secondary or sense winding may be substantially solenoidal and have a turns density which is substantially uniform along the axial extent of said at least one secondary or sense winding.
In another aspect, said co-pending application also provides a sensing system comprising a sensor having a drive or exciting winding for establishing a forward or drive field, at least one secondary or sense winding in which a voltage may be induced in the presence of said forward or drive field, and at least one conductive screen within which eddy currents are generated in the presence of said forward or drive field to establish a counter-field opposing said forward or drive field, said at least one conductive screen and said at least one secondary or sense winding being displaceable relative to one another within said forward or drive field so that said at least one secondary or sense winding may be shaded by said at least one conductive screen to a varying extent to thereby vary the voltage induced in said at least one secondary or sense winding, said at least one conductive screen having an axis of symmetry and a surface region which extends substantially circumferentially with respect to said axis of symmetry, said at least one secondary or sense winding being disposed about the periphery of a notional cylinder which is substantially coaxial with said axis of symmetry of said at least one conductive screen, and the relative displacement of said at least one conductive screen and said at least one secondary or sense winding being in the direction of the axis of symmetry of said at least one conductive screen and said axis of said notional cylinder which is substantially coaxial therewith, means for applying an alternating voltage to said drive or exciting winding, and means for processing the output signal from said at least one secondary or sense winding to provide a signal indicative of the relative disposition of said at least one conductive screen and said at least one secondary or sense winding.
The basic elements of a screened inductance transducer are, therefore, at least two, normally stationary, coils and a passive, conductive screen, whose position controls the mutual coupling between the coils. The basic screened inductance effect has, surprisingly, found no widespread use in current commercial position measurement systems. It has obvious advantages of simplicity, in particular because of the passive nature of the moving element.
Advantages of screened inductance sensors include:
(a) totally brushless operation, PA0 (b) ease of design for different applications, PA0 (c) simplicity of the materials used in sensor construction, and PA0 (d) ease with which high volume manufacturing techniques, such as injection moulding, can be used in sensor construction. PA0 (a) evaluating a winding shape for a two pole planar linear geometry for specified requirements of the winding. PA0 (b) transforming said two pole planar linear geometry of winding shape to a winding shape appropriate to specified requirements of the sensor, and PA0 (c) outputting said transformed winding shape to provide printed circuit artwork. PA0 (a) defining a single-turn winding for a planar linear geometry, said single-turn winding having a width which varies in said single direction of relative displacement of said at least one conductive screen and said single-turn winding so that the area swept out by notional relative displacement of said at least one conductive screen and said single-turn winding in a direction equivalent to said single direction of relative displacement follows a specified transfer function, PA0 (b) transforming said planar linear geometry of said single-turn winding to a winding shape appropriate to specified requirements of the sensor, and PA0 (c) outputting the transformed winding shape to provide printed circuit artwork. PA0 (a) defining a multi-turn winding for a planar linear geometry, said multi-turn winding having a shape enabling the area swept out by notional relative displacement of said at least one conductive screen and said multi-turn winding in a direction equivalent to said single direction of relative displacement to follow a specified sensor transfer function. PA0 (b) transforming said planar linear geometry of said multi-turn winding to a winding shape appropriate to specified requirements of the sensor, and PA0 (c) outputting the transformed winding shape to provide printed circuit artwork. PA0 (a) means for defining a winding shape for a two pole planar linear geometry for specified requirements of the winding, PA0 (b) means for transferring said two pole planar linear geometry of winding shape to a winding shape appropriate to specified requirements of the sensor, and PA0 (c) outputting said transformed winding shape to provide printed circuit artwork. PA0 (a) defining a winding shape for a two pole planar linear geometry for specified requirements of the winding, PA0 (b) transferring said two pole planar linear geometry of winding shape to a winding shape appropriate to specified requirements of the sensor, and PA0 (c) outputting said transformed winding shape to provide printed circuit artwork for said winding.
The latter two points in particular make the screened inductance sensor attractive for use in cost conscious areas such as the automotive industry, for example, as a fuel gauge.
A variety of screened inductance sensors especially suited to liquid level measurement and having screen/winding sealing barriers been described in a further co-pending patent application with particular reference to the use of stainless steel as the barrier material. According to said further co-pending application, there is provided a screened inductance sensor in which barrier means are interposed between the windings and the screen, so that the screen may be disposed within a region environmentally isolatable from the region in which the windings are located.
Said barrier means may be defined by a wall portion and may be formed from stainless steel. The frequency of an energising input to be applied to the drive winding is selected in dependence of the thickness of the wall for the drive field to penetrate said barrier means or wall for the shading of the secondary winding by the screen. The screen may also be at least in part of a relatively poorly conductive material, such as stainless steel.
In a further aspect, said further co-pending application discloses a screened inductance sensor in which the screen is at least in part of a relatively poorly conductive material, such as stainless steel. The barrier means and the screen may be of the same material, such as stainless steel, to contend with the environment in which the device is employed, in which case the screen has a thickness dimension substantially in excess of the thickness of the barrier means.
The use of relatively poorly conductive materials for both the screen and the barrier entails the establishment of an appropriate relativity between their respective thicknesses. That of the barrier means is generally determined by structural demands on the sensor, and the frequency of the energising input to the drive winding is then selected to provide a skin depth sufficient for the passage of the drive field. By contrast, the thickness of the screening element is selected precisely to block the field and thus secure the required screening function. The skin depth determines the extent to which a magnetic field penetrates a material. As conductivity rises, so does field penetration decrease. The appropriate balance of material thicknesses may be selected by calculation from the various parameters of the sensor system and the characteristics of the materials used.
In an especially favoured construction, there is provided according to said further co-pending application a screened inductance level transducer in which the screen has a substantially cylindrical outer peripheral region and the secondary or sense winding is disposed about the periphery of a notional cylinder substantially concentric with said peripheral region of the screen, and said screen and said secondary or sense winding are displaceable with respect to one another in an axial direction relative to the substantially common axis of said screen peripheral region and said notional cylinder, said barrier means being defined by a tube within which said screen is axially displaceable, and the drive and sense windings being disposed externally of said tube. Preferably the screen is at least in part of stainless steel and it may consist wholly of stainless steel. Alternatively the screen may be formed from copper or copper encased or sheathed in stainless steel. In a favoured construction, said drive winding is also disposed about the periphery of a further notional cylinder, again substantially concentric with the common axis of said peripheral region of the screen and said first-mentioned notional cylinder.